The prevailing cultural esthetic among fair-skinned persons in many countries and places is to have a skin which is tanned, i.e., which has a light yellowish brown or even deeper brown color produced by exposure to the sun or to an artificial source of ultraviolet light. The tanning of the skin is produced as a result of the darkening of preformed melanin, accelerated formation of new melanin, and retention of melanin in the epidermis as a result of retardation of keratinization. The darkening of existing melanin, possibly caused by oxidation and referred to as the Meirowsky phenomenon, begins within a few seconds of exposure to long-wave ultraviolet light and is complete within minutes up to a few hours, depending on the individual involved.
For those many individuals who wish to achieve a tanned skin, the most readily available means for doing so is by exposure of their skin to natural sunlight. However, this method carries with it certain hazards, chief among which is the risk of sunburn, i.e., actual injury to the skin produced by excessive exposure to ultraviolet rays. The injury is accompanied by erythema, tenderness, and sometimes blistering. Furthermore, excessive exposure to ultraviolet radiation is considered by the medical community to be a leading factor in the oncogenesis of melanomas and other skin cancers, as well as an accelerating agent in the aging of the skin, particularly its tendency to sag and wrinkle. In order to mitigate or prevent such excessive exposure, such individuals will usually seek the protection of various sunscreen products, which contain sunscreen agents that act either by absorbing the ultraviolet radiation or by reflecting incident light. Sunscreen products also provide the user with other protections and benefits which are desirable, such as those available from skin moisturizing, protective and healing compounds, anesthetic and anti-inflammatory agents, etc. Certain embodiments of the present invention utilize such conventional sunscreen materials in order to achieve a product with the most universal appeal and with the greatest possible number of benefits to its users.
Where ultraviolet radiation is the source of skin tanning, it should be noted briefly that such electromagnetic radiation lies beyond the violet end of the spectrum, whence its name. Ultraviolet radiation lies between violet rays and roentgen or "X" rays, is characterized by wavelengths between 200 and 400 nm, and has powerful actinic and chemical properties. Over 99% of ultraviolet radiation has wavelengths between 320 and 400, and is referred to as ultraviolet A, or simply UVA. The remaining 1% of ultraviolet radiation comprises ultraviolet B, or UVB radiation, which has wavelengths between 290 and 320 nm. UVB causes sunburn and a number of damaging photochemical changes within cells, including damage to DNA, leading to premature aging of the skin, premalignant and malignant changes, and a variety of photosensitivity reactions. Wavelengths between 200 and 290 nm characterize ultraviolet C radiation, or UVC, all of which is filtered out by the ozone layer and does not reach the earth's surface. Many persons, in order to achieve a satisfactorily tanned skin but unable to take advantage of naturally occurring sunlight, have resorted to the use of artificial sources of ultraviolet light, which are also much easier to regulate with respect to the amount of ultraviolet radiation to which the skin is exposed, than is the case with natural sunlight. On the other hand, there are many persons who, for a variety of reasons, make use of so-called self-tanning or sunless product formulations in order to achieve a satisfactory tan.
For example, many persons have skin complexions which do not tan readily or evenly when exposed to sunlight, and many others suffer significant adverse side effects from sun exposure, including severe sunburn. For such people, once a tan is obtained, there is a great deal of interest in maintaining or extending the life of that tan, which will naturally dissipate over time as the layers of skin which actually comprise the tan become necrotic and are eventually sloughed off. All such persons would greatly benefit from access to any means of obtaining a tanned skin, or at least a skin having the appearance of a naturally tanned skin, which did not require being exposed to sunlight, with all of the attendant adverse effects which arise from such exposure to ultraviolet radiation. Accordingly, such persons in ever greater numbers have turned to so-called self-tanning or sunless product formulations in order to achieve a "tan" without going through the harsh medium of the sun. However, there are many other persons who, although not as severely affected by exposure to ultraviolet radiation, also have turned to self-tanning products as a way of achieving a more uniform tan, of obtaining a deeper tan with significantly less total exposure to sunlight, and of extending the natural life of the tan which they have acquired for themselves. For such persons, a suitable product is to be found in those embodiments of the present invention which combine the components of a self-tanning composition with the components of a sunscreen formulation.
Self-tanning agents and formulations of various types and compositions are known in the art. For example, sunless tanning agents which have been discovered and used heretofore include dihydroxyacetone (DHA), glyceraldehyde (glycerol aldehyde) and related alcohol-aldehydes, various indoles and imidazoles and their derivatives, and pigmentation agents approved for use with humans, e.g., methoxsalen and trioxsalen. Dihydroxyacetone in particular is currently the self-tanning agent most widely used, and it has been known in the art for some time as an artificial tanning agent for use on the human epidermis. As used hereafter, the term "dihydroxyacetone" will be used to mean not only that specific composition of matter, but all of the other species described further herein which may be substituted for dihydroxyacetone in accordance with the objectives herein enumerated, whether with better or poorer results as measured by the several criteria also further below-described in more detail.
It has been widely postulated in the art that dihydroxyacetone achieves this effect by reacting with various skin proteins and amino acids to produce a tan-colored reaction product. Dihydroxyacetone has been used in a number of self-tanning formulations over the years combined, e.g., with colorants and sunscreen agents. Dihydroxyacetone, which may also be named as 1,3-dihydroxy-2-propanone, has the following structural formula: ##STR1##
which corresponds to a elementary formula of C.sub.3 H.sub.6 O.sub.3 and a molecular weight of 90.08. Dihydroxyacetone is a crystalline powder which is fairly hygroscopic and has a melting point of about 75-80.degree. C. It has a characteristic odor. The normal form in which dihydroxyacetone exists is a dimer, which is slowly soluble in 1 part water and 15 parts ethanol. However, when dihydroxyacetone is freshly prepared it reverts rapidly to the monomer form, which is very soluble in water, ethanol, ether and acetone.
As already mentioned, dihydroxyacetone is probably the most wisely used commercially of all self-tanning agents, and it is the key active ingredient in numerous self-tanning products which have achieved solid market success. The precise mechanism by which dihydroxyacetone causes the self-tanning reaction was studied by Bobin et al. and reported in J. Soc. Cosmet. Chem., 35, 265-272, August 1984. They studied the Maillard reaction of dihydroxyacetone with various amino acids found naturally in the hydrolipid pellicle and first layers of the stratum corneum of the skin. The Maillard reaction is one between the amino groups of amino acids, peptides or proteins with the "glycosidic" hydroxyl group of sugars, ultimately resulting in the formation of brown pigments. This reaction is also referred to commonly as the "browning" reaction, since this same reaction takes place when various foodstuffs are browned by heating during the process of their preparation for eating.
However, despite such wide use and market acceptance, dihydroxyacetone suffers from several notable drawbacks which have both prevented products containing it from obtaining even wider market penetration, and precluded the production of a cosmetic product formulation with all of the optimum characteristics desired by the ultimate consumer. One of these notable drawbacks is an unacceptably short shelf life. Dihydroxyacetone possesses a number of chemical and physical properties which place significant inherent constraints on its use. Dihydroxyacetone in the monomer form is highly soluble in water, which thereby exposes it to the action and affects of various agents, transports it into many unintended environments, and makes its isolation difficult. The substantial oxidizing capacity of dihydroxyacetone, which is the ultimate mechanism by which it presumably operates to produce a "tan", is a serious problem facing the cosmetic formulator or other artisan, since it can lead not only to undesired activity once applied to the skin of the ultimate consumer, but can also lead to difficulties in terms of its interaction with the other components with which it is formulated. Further, dihydroxyacetone does not possess a stable pH in solution, but is, rather, subject to a shifting pH which makes its formulation and ultimate use environment problematic to manage. Further still, dihydroxyacetone is especially sensitive to the degradative activities of bacteria and other microorganisms, against the attacks of which it must be protected, both in its inceptive formulation, as well as in its eventual use environment.
A major result of the above-described properties of dihydroxyacetone is its degradation upon standing, which is likely to take place while the product containing it is in the possession of the ultimate consumer, if not even before purchase, while still in transit. This abbreviated shelf life has required heretofore that products containing dihydroxyacetone be used soon after preparation, and that said products be stored under conditions optimized with regard to temperature and relative humidity so that the progress of degradation could at least be delayed. Nevertheless, the proportion of dihydroxyacetone remaining active in said products has been found to rapidly diminish with time. In addition to this loss of activity, the result of the chemical and physical degradation of dihydroxyacetone is the production of undesirable discoloration as well as the formation of disagreeable odors.
Another significant drawback of dihydroxyacetone, which is not, however, related to its instability, is the inherent and unpleasant odor which results from the reaction of dihydroxyacetone and those components of the skin with which it comes in contact during the self-tanning process. Accordingly, in order to address and overcome all of the above-described challenges posed by the properties of dihydroxyacetone, the artisan of ordinary skill in preparing cosmetic and therapeutic products has been forced to exercise all of the formulation skills possessed by said artisan. The result, however, has not been the discovery of a totally successful product up until the time that the present invention was made as described further herein.